A vehicle crash sensing system detects and discriminates severe crash events which require actuation of a safety device (e.g., deployment of an airbag) such as those listed in Table 1, from minor crash incidents which do not, such as those listed in Table 2.
TABLE 1 ______________________________________ Type of Collision Speed (km/h) ______________________________________ Full frontal to barrier 20-48 30.degree. angle to barrier 32-48 On-center pole 24-48 Full frontal to rear of parked car 96 ______________________________________
TABLE 2 ______________________________________ Type of Collision Speed (km/h) ______________________________________ Undercarriage Hit 32 Car-to-Deer 80 Square Block Road 64 Chatter Bumps 96 Hood Slams N/A Door Slams N/A Hammer Blows (5-8 lbs.) N/A ______________________________________
Discrimination is accomplished by means of a vehicle-mounted accelerometer and an associated signal processing algorithm contained within a microprocessor. Since the total available time for deploying an airbag to effectively restrain occupants in a severe crash event is very short, the ability to quickly and reliably determine the severity of a collision is paramount. Equally important is the system's immunity to inadvertent deployment during minor crash incidents.
Many prior art airbag deployment algorithms have been developed which utilize one or more quantities for measuring the severity of a collision. These "quantities" or "parameters" have included vehicle velocity change, energy, power, power rate, jerk, predicted occupant movement, as well as energy and oscillation measure of the vehicle deceleration signal. The value of these quantities are generally calculated as a function of successively sampled accelerometer data. Based upon test data obtained from the accelerometer during a representative set of minor crash incidents, one or more boundary thresholds are set. Airbag deployment is initiated whenever the values of some or all of these quantities exceed their respective boundary threshold levels.
A major problem in quickly discriminating the severity of a collision is the relatively soft initial impact of some severe crash events which is similar to that of minor crash incidents. For example, referring to FIG. 1, a 30 mph (48 km/h) center pole impact 10 (characterized as a severe crash event necessitating deployment of an airbag) has a similar velocity change signal trace as a 9 mph (14 km/h) frontal barrier impact 12 (characterized as a minor crash incident). Not until after 58 msec into the crash (point 13) are the severity of the two events distinguishable. A second example is depicted in FIG. 2 in which the vehicle velocity change signal trace of a 48 km/h, 30.degree. angle impact 14 (characterized as a severe crash event) is very similar to that of an 80 km/h simulated deer hit 16 (characterized as a minor crash incident). In this case, absolute discrimination is not possible until 25 msec into the crash (point 17). Any attempts to distinguish between the severity of the two crash events of FIGS. 1 and 2 prior to the last point of intersection (13 and 17) of their respective signal traces will compromise the system immunity to deployment during minor crash incidents.
In view of the foregoing, there is a desire to develop a method for reducing the time required for distinguishing the severity of a crash event for controlling actuation of a vehicle safety device, without compromising system immunity to minor crash incidents.